Hydraulic system for an automatic transmission of a motor vehicle

ABSTRACT

A hydraulic system for an automatic transmission, in particular a dual-clutch transmission, of a motor vehicle. A high-pressure circuit, in which a pressure accumulator, at least one clutch, and gear selectors are connected, and a low-pressure circuit for cooling the clutch. The high-pressure circuit and the low-pressure circuit have at least one hydraulic pump, which can be driven by an electric motor. The hydraulic system also has a control unit, which activates the electric motor of the hydraulic pump when a requirement to charge the pressure accumulator is identified. The high-pressure and low-pressure circuits are connected via a bypass line to an integrated accumulator charging valve, which in a non-charging position fluidically connects the hydraulic pump to the low-pressure circuit and in a charging position fluidically connects the hydraulic pump to the high-pressure circuit.

FIELD

The invention relates to a hydraulic system for an automatictransmission, more particularly a dual-clutch transmission, of a motorvehicle.

BACKGROUND Summary

In a dual-clutch transmission, two sub-transmissions enable fullyautomatic gear changes without any interruption of tractive power.Torque is transmitted via one of two clutches, which connects the twosub-transmissions to the drive. The clutches and the gear selectors forengaging the gears are actuated via hydraulic cylinders, which arecontrolled hydraulically via a hydraulic system.

From DE 10 2014 003 083 A1 a generic hydraulic system is known, whichhas a pressure accumulator for supplying an accumulator pressure in thehydraulic system. In a clutch path leading from the pressure accumulatorto the clutch hydraulic cylinder, a control valve is positioned, whichcan be controlled by an electronic control unit and can be used toadjust the hydraulic pressure applied to the clutch hydraulic cylinder.The control unit is preferably assigned a pressure sensor (DE 10 2013003894 A1), with which the hydraulic pressure applied to the clutchhydraulic cylinder can be detected. The hydraulic system also includes ahydraulic charge pump, which delivers hydraulic fluid into the hydraulicsystem in a charging operation in order to increase the accumulatorpressure.

The high-pressure and low-pressure circuits of the hydraulic system canbe connected via a bypass line to an integrated accumulator chargingvalve. In a non-charging position, the accumulator charging valve canfluidically connect the hydraulic pump to the low-pressure circuit,while at the same time the hydraulic pump is decoupled from thehigh-pressure circuit. Conversely, in a charging position, theaccumulator charging valve can fluidically connect the hydraulic pump tothe high-pressure circuit, while at the same time the hydraulic pump isdecoupled from the low-pressure circuit. The accumulator valve canswitch automatically from the charging position to the non-chargingposition at a first switchover time, specifically when the accumulatorpressure in the high-pressure circuit exceeds an upper pressurethreshold value. Conversely, the accumulator charging valve can switchautomatically from the non-charging position to the charging position ata second switchover time, when the accumulator pressure drops below alower pressure threshold value.

In the prior art, complex sensor systems are required to detect amalfunction of the accumulator charging valve. Such a malfunction mayoccur, for example, when a spring in the spring-loaded accumulatorcharging valve breaks or when, for example due to soil deposits, thetravel path of the accumulator charging valve is impeded. In that case,the risk exists that the accumulator charging valve may no longer switchbetween the charging position and the non-charging position at plausiblelower/upper pressure threshold values. Highlighted as relevantparameters for the functioning of the accumulator charging valve arewhat is known as the valve spread, which is the difference in pressurebetween the upper and lower pressure threshold values, and theswitchover times between the charging and non-charging positions.

The object of the invention is to provide a hydraulic system in whichthe operational reliability of the accumulator charging valve can beensured with reduced sensor system complexity.

The control unit includes a diagnostic module, with which a valve spreaddiagnosis is performed, in which an actual valve spread between thelower and upper pressure threshold values is determined. The diagnosticmodule may have an analysis unit which compares the determined actualvalve spread with a target valve spread. If a significant deviation isfound, the analysis unit can diagnose a fault.

The valve spread diagnosis may be preceded by a switchover timingdiagnosis, which can likewise be carried out by the diagnostic module.In the switchover timing diagnosis, it is determined whether the actualaccumulator pressure lies within range of the upper pressure thresholdvalue at the first switchover time. Alternatively and/or additionally,it can be determined whether the actual accumulator pressure is withinrange of the lower pressure threshold value at the second switchovertime. If the actual accumulator pressure detected at the firstswitchover time is found to deviate significantly from the upperpressure threshold value, a fault is diagnosed. Conversely, if theactual accumulator pressure detected at the second switchover time isfound to deviate significantly from the lower pressure threshold value,this is likewise diagnosed as a fault.

In a technical implementation, the hydraulic system can have at leastone clutch path which leads from the pressure accumulator to the clutchhydraulic cylinder. In the clutch path, a clutch valve can bepositioned, which can be controlled by the control unit and can be usedto adjust the hydraulic pressure applied to the clutch hydrauliccylinder. The electronic control unit can be assigned a pressure sensor,which can detect the hydraulic pressure applied to the clutch hydrauliccylinder. In a simple technical implementation, the pressure sensorpositioned in the clutch path can be used for detecting the actualaccumulator pressure during the switchover timing diagnosis. Duringnormal driving operation, the pressure sensor positioned in the clutchpath performs a safety function, in which it monitors whether the clutchis depressurized or pressurized. During the switchover timing diagnosis,the clutch path pressure sensor can also be used in a dual function todetect the actual accumulator pressure.

In light of its above safety function during normal driving operation,the clutch path pressure sensor is designed as having a correspondinglysmall measuring range (in other words, as economical). The measuringrange of the pressure sensor may therefore lie outside of, i.e. below,the upper pressure threshold value at which the accumulator chargingvalve switches automatically from its charging position to itsnon-charging position. In that case, the actual accumulator pressuretherefore cannot be detected immediately by the clutch path pressuresensor at the first switchover time. It is therefore preferable for ananalysis unit to estimate, on the basis of measured pressure values thatare within the pressure sensor measuring range, a time frame withinwhich the first switchover time and/or the second switchover time willlie if the accumulator charging valve is functioning properly. If theanalysis unit determines that the first/second switchover time liesoutside of this time frame, a fault will be diagnosed. The analysis unitmay have an extrapolation component, for example, which extrapolates theaforementioned time frame based upon the aforementioned measuredpressure values.

If no fault is detected in the above switchover timing diagnosis, thevalve spread diagnosis may follow as a follow-on diagnosis.

To determine the actual valve spread, the analysis unit can specify adiagnosis time interval, which begins at the first switchover time andends at the second switchover time (i.e., during a non-chargingoperation). In the diagnosis time interval, an accumulator pressuredecrease corresponding to the actual valve spread occurs as a result ofactuation of a reference hydraulic cylinder and as a result of hydraulicsystem leakage. The procedure for determining this accumulator pressuredecrease during the diagnosis time interval is preferably as follows:The reference hydraulic cylinder is equipped with a position sensor,which detects the piston travel distances occurring with gear selectoractuations. The diagnostic module can integrate the piston traveldistances during the diagnosis time interval to obtain a totaldiagnostic distance, and can then use this total to calculate thepressure decrease associated with the gear selector actuations. From thesum of the pressure decrease associated with the gear selectoractuations plus the leakage-induced pressure decrease, the analysis unitcan then determine the accumulator pressure decrease during thediagnosis time interval. The leakage-induced pressure decrease duringthe diagnosis time interval can be determined based upon previousdiagnoses or leakage measurements.

The first and second switchover times during the change between thecharging and non-charging positions can be determined as follows: Thecontrol unit may be assigned a current measuring device, with which anactual current consumption of the electric motor can be detected. Thecontrol unit can specify the time at which the system changes from highcurrent consumption to low current consumption as the first switchovertime. Conversely, the control unit can specify the time at which thesystem changes from low current consumption to high current consumptionas the second switchover time.

In a preferred embodiment, the reference hydraulic cylinder can be agear selector hydraulic cylinder that has been found during a precedinggear selector diagnosis to be fault-free. In a reference gear selectorpath leading from the pressure accumulator to the reference hydrauliccylinder, a control valve can be positioned, which can be controlled bythe control unit and can be used to adjust the hydraulic pressureapplied to the reference hydraulic cylinder.

The switchover timing diagnosis and/or the valve spread diagnosis arepreferably performed as follow-on diagnoses that are preceded by apreload pressure and accumulator volume diagnosis in the pressureaccumulator, a clutch path diagnosis, and a gear selector pathdiagnosis.

In that case, an accumulator volume diagnosis can preferably be carriedout by means of the diagnostic module. The accumulator volume diagnosisbegins with the pressure accumulator first being filled completely withhydraulic fluid in a diagnostic charging operation. The diagnosticmodule then selects one of the gear selector hydraulic cylinders as thereference hydraulic cylinder. This cylinder is actuated during adiagnosis time interval, inducing a removal of hydraulic fluid, whichresults from an intermittent actuation of the reference hydrauliccylinder (i.e., displacement volume) and the hydraulic system leakage.The reference hydraulic cylinder is actuated during the diagnosis timeinterval until an ambient pressure prevails in the hydraulic system dueto the associated removal of hydraulic fluid. At ambient pressure, theoil chamber in the pressure accumulator is completely drained, i.e., thepressure piston is pressed with a preload force against a stop of thepressure accumulator.

The diagnostic module has an analysis unit, which determines the abovehydraulic fluid removal and compares this with a reference pressureaccumulator volume. If a significant deviation is found, an accumulatorvolume fault is diagnosed.

In one technical implementation, the reference hydraulic cylinder may beequipped with a position sensor, which detects the piston traveldistances occurring in the reference hydraulic cylinder with gearselector actuations. During the accumulator volume diagnosis, thediagnostic module can integrate the piston travel distances to obtain atotal travel distance and can then use this total to calculate thehydraulic fluid removal (i.e., the displacement volume) associated withthe gear selector actuations.

In one simple embodiment variant, the existence of ambient pressure inthe hydraulic system can be detected with the aid of the position sensoras follows: When the ambient pressure is reached in the hydraulicsystem, the reference hydraulic cylinder is no longer pressurized by anactuating pressure that leads to a piston displacement. The positionsensor therefore detects that no further piston travel is occurring inthe reference hydraulic cylinder. From this, the diagnostic moduleconcludes that the ambient pressure has been reached and the diagnosistime interval has ended. In the above accumulator volume diagnosis, inaddition to the removal of hydraulic fluid resulting from the actuationof the reference hydraulic cylinder (hereinafter referred to asdisplacement volume), the removal of hydraulic fluid associated withconstant hydraulic system leakage must be considered. This canpreferably already be stored in the diagnostic module from previousmeasurements and/or diagnoses.

In addition to the aforementioned reference gear selector path, thehydraulic system has at least one clutch path which leads from thepressure accumulator to the clutch hydraulic cylinder and in which aclutch valve that can be controlled by the electronic control unit ispositioned. The clutch valve can be used to adjust the hydraulicpressure applied to the clutch hydraulic cylinder. The electroniccontrol unit is also assigned a pressure sensor, which can detect thehydraulic pressure applied to the clutch hydraulic cylinder.

During the accumulator volume diagnosis, both the above-described clutchpath and the reference gear selector path leading to the referencehydraulic cylinder can be pressurized with the actual accumulatorpressure prevailing in the hydraulic system. The actual accumulatorpressure profile can thereby be detected during the accumulator volumediagnosis in a metrologically simple manner. In addition, with thishydraulic system structure, the leakage behavior of the clutch path andof the reference gear selector path can be carried out using thepressure sensor located in the clutch path. In contrast to the referencehydraulic cylinder, which is located in the reference gear selectorpath, the hydraulic cylinders of the other gear selector paths aredecoupled from the accumulator pressure, i.e., they are not pressurizedwith accumulator pressure.

The accumulator volume diagnosis can preferably be carried out as afollow-on diagnosis following a gear selector path diagnosis. In thatcase, the accumulator volume diagnosis can be performed only under thecondition that in the preceding gear selector path diagnosis, at leastone non-malfunctioning gear selector has been identified, which can beused as a reference gear selector in the accumulator volume diagnosis.

It is possible for the aforementioned gear selector path diagnosis to becarried out using the diagnostic module, in which case the diagnosticmodule, using the pressure sensor located in the above at least oneclutch path, checks the leakage behavior in the respective gear selectorpath. The gear selector path diagnosis can preferably be carried out asa follow-on diagnosis following the aforementioned clutch pathdiagnosis. The gear selector path diagnosis is preferably carried outsolely under the condition that at least one clutch path with fault-freeleakage is identified in a preceding clutch path diagnosis, which willbe described later. In that case, the pressure sensor in the clutch paththat is identified as fault-free (hereinafter referred to as thereference clutch path) is used for the subsequent gear selector pathdiagnosis.

For the gear selector path diagnosis, the diagnostic module opens theclutch valve located in the reference clutch path so that the pressuresensor located in the reference clutch path can detect an actualaccumulator pressure profile. The diagnostic module also opens apressure control valve located in a connecting line leading to the gearselectors, in order to establish a pressure connection between thepressure sensor located in the reference clutch path and the gearselector valve located in the gear selector path.

In a first diagnostic step, a diagnostic charging operation isperformed, in which the actual accumulator pressure detected by thepressure sensor is increased to an upper threshold value, at which thehydraulic charge pump is switched off. Once the diagnostic chargingoperation is completed, a third analysis unit can detect a pressuregradient of the accumulator pressure profile via the pressure sensor,which it can compare with a reference pressure gradient and analyzewhether a fault-free or a faulty pressure decrease (i.e., gear selectorleakage) is present in the accumulator pressure profile.

In one technical implementation, the hydraulic system can comprise aplurality of gear selector paths connected to one another in parallel,in each of which a gear selector valve is located, which can be adjustedbetween a closed valve position and two flow-through valve positions.

In such a configuration, the gear selector path diagnosis can beperformed for each of the flow-through valve positions separately in thegear selector path to be tested and analyzed to identify faults. Incontrast, all the gear selector valves in the gear selector paths notbeing tested are switched to the closed valve position, in order toincrease measuring accuracy in the gear selector path being tested.

The aforementioned detection of the pressure gradient in the accumulatorpressure profile is carried out within a measuring time interval. Thestart time of said interval is preferably immediately after completionof the diagnostic charging operation. During measurement of the pressuregradient, the actual accumulator pressure at the start time and at themeasurement end time of the measuring time interval is also detected.Based upon these two absolute pressure values, if a sufficiently largeaccumulator pressure differential between the start time and the endtime exists, the diagnostic module can make a fault-free diagnosis.

The pressure accumulator of the hydraulic system can be structured as apiston-cylinder unit which has an oil chamber connected to the clutchpath and a preloaded pressure piston, to which a preload pressure isapplied. The preloading is achieved, for example, by a gas pressure oralternatively by a spring. When the oil chamber is completely drained,the pressure piston is pressed with a preload force against a mechanicalstop in the pressure accumulator. In such a completely drained state,the clutch path is not pressurized. Rather, in this case ambientpressure prevails in the clutch path. In the prior art, complex sensorsystems are required to detect a malfunction of the pressureaccumulator, for example, a reduction in gas pressure due to gasleakage.

The gear selector path diagnosis can preferably be carried out as afollow-on diagnosis following a preload pressure diagnosis and/or aclutch path diagnosis.

In light of the above, the diagnostic module can preferably be used tocheck the preload pressure of the pressure accumulator. For thispurpose, at least one or more reference values that represent areference accumulator pressure profile over time during a chargingprocess are stored in the diagnostic module. For the pressureaccumulator diagnosis, a charging operation is performed, in which theclutch valve located in the clutch path is opened all the way, so thatthe pressure sensor can detect an actual accumulator pressure profileover time during the charging operation. To analyze the actualaccumulator pressure profile over time, the diagnostic module has ananalysis unit, with which a pressure accumulator fault can be diagnosedif a significant deviation is detected between the reference accumulatorpressure profile and the actual accumulator pressure profile.

For the pressure accumulator diagnosis, the charging operation isperformed at a constant charging speed of the hydraulic charge pump. Theclutch path is thereby filled with hydraulic fluid, specifically until apreload pressure time at which the hydraulic pressure detected by thepressure sensor (i.e., the actual accumulator pressure) is as great asthe (actual) preload pressure of the pressure accumulator. As thecharging process continues, the oil chamber of the pressure accumulatoris filled starting from the preload pressure time, specifically withdisplacement of the pressure piston and with a further increase in theactual accumulator pressure.

Such a charging process results in a characteristic time-charge curve.This curve can extend between a diagnosis start time, at which thepressure accumulator oil chamber is completely drained, and theaforementioned upper threshold value, and can be used for the pressureaccumulator diagnosis as follows: The time-charge curve (i.e., theactual accumulator pressure profile) has a steep pressure gradient untilthe preload pressure time is reached and has a dramatically reducedpressure gradient in comparison after the preload pressure time. If thepressure accumulator is functioning properly, the actual preloadpressure detected at the preload pressure time will coincide with thestructural interpretation of the pressure accumulator preload pressurewhich is stored in the diagnostic module, taking into account the strongtemperature dependency.

In the analysis unit of the diagnostic module, the actual accumulatorpressure detected by the pressure sensor at the preload pressure time iscompared with the predefined reference preload pressure of the pressureaccumulator. If a significant deviation between the two values is found,an implausible preload pressure in the pressure accumulator will beconcluded.

As was already mentioned above, diagnosis is started under thediagnosis-starting condition that the oil chamber of the pressureaccumulator is fully drained, and an ambient pressure prevails in thehydraulic system. To achieve this diagnosis-starting condition, at leastone hydraulic cylinder of the clutch and/or the gear selector isactuated in advance until, as a result of the removal of hydraulic fluidassociated with the hydraulic cylinder actuation, the actual accumulatorpressure detected by the pressure sensor is reduced to ambient pressure.In that case, the pressure accumulator oil chamber is also automaticallycompletely drained.

To determine the preload pressure time, the analysis unit can analyzeand compare the pressure gradients over time before and after thepreload pressure time and can use the result of this comparison tocalculate the preload pressure time or to determine whether or not apressure accumulator fault exists.

A dual-clutch transmission has two clutches, which are connected to thepressure accumulator via substantially identical clutch paths. In thatcase, the above-described pressure accumulator diagnosis can be carriedout twice, specifically as part of a first partial diagnosis using thepressure sensor located in the first clutch path and with the clutchvalve in the second clutch path closed, and as part of a second partialdiagnosis using the pressure sensor located in the second clutch pathand with the clutch valve in the first clutch path closed. In theanalysis unit, the pressure accumulator diagnosis is verified by acomparison of the first and second partial diagnoses. If identicalpressure accumulator faults are found in the first and the secondpartial diagnosis, the analysis unit will diagnose a pressureaccumulator fault. In contrast, if different fault results are found inthe two partial diagnoses, the analysis unit will diagnose a fault(i.e., a leak, for example) in one of the two clutch paths.

In a further embodiment, the diagnostic module can additionally carryout its own clutch path diagnosis, which follows the preload pressurediagnosis immediately as a follow-on diagnosis. For the clutch pathdiagnosis, the diagnostic charging operation carried out during thepreload pressure diagnosis is continued until a maximum accumulatorpressure (i.e., the upper threshold value) is reached, and is endedthere at a switch-off time. When the charging operation has ended, theanalysis unit compares the further actual accumulator pressure profilewith stored reference values and analyzes whether a fault-free or afaulty, leakage-induced pressure drop is present in the accumulatorpressure profile (detected by the pressure sensor).

It is preferable for the above clutch path diagnosis to be performedonly as long as a fault-free preload pressure in the pressureaccumulator is ensured. Thus, the clutch path diagnosis can preferablybe performed in the diagnostic module only if a fault-free pressureaccumulator preload pressure exists during the preload pressurediagnosis.

The advantageous embodiments and/or refinements of the inventiondescribed above and/or reflected in the dependent claims may be usedindividually or in any desired combination with one another except, forexample, in the case of clear dependencies or incompatible alternatives.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention and its advantageous embodiments andrefinements along with the advantages thereof will be described ingreater detail with reference to the drawings.

In the drawings:

FIG. 1 is a block diagram of a dual-clutch transmission for a motorvehicle having seven forward gears and one reverse gear;

FIG. 2a shows a block diagram of a hydraulic system of a dual-clutchtransmission and a rough schematic of the structure of a pressureaccumulator;

FIG. 2b shows another block diagram of a hydraulic system of adual-clutch transmission and a rough schematic of the structure of apressure accumulator;

FIG. 3 is a block diagram showing the program blocks required forpressure accumulator and clutch path diagnosis in a diagnostic module;and

FIG. 4 contains graphs illustrating the pressure accumulator and clutchpath diagnosis;

FIG. 5 is a block diagram showing the program blocks required for gearselector path diagnosis in the diagnostic module;

FIG. 6 contains graphs illustrating the gear selector path diagnosis;

FIG. 7 is a block diagram showing the program blocks required foraccumulator volume diagnosis in the diagnostic module;

FIG. 8 contains graphs illustrating the accumulator volume diagnosis;

FIG. 9 is a block diagram showing the program blocks required forswitchover timing diagnosis in the diagnostic module;

FIG. 10 is a block diagram showing the program blocks required for valvespread diagnosis in the diagnostic module;

FIG. 11 contains graphs illustrating the profiles over time duringswitchover timing diagnosis and during valve spread diagnosis;

FIG. 12 is a block diagram showing the program blocks required forsafety valve diagnosis in the diagnostic module;

FIG. 13 contains graphs illustrating the profiles over time of relevantparameters during safety valve diagnosis;

FIG. 14 is a block diagram showing the program blocks required fordelivery volume flow diagnosis in the diagnostic module;

FIG. 15 contains graphs illustrating the profiles over time duringdelivery volume flow diagnosis; and

FIG. 16 shows an analysis unit into which the fault signals generated inthe fault memories can be read.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a dual-clutch transmission for a motorvehicle with all-wheel drive. This dual-clutch transmission has sevenforward gears (see circled numerals 1 through 7) and one reverse gearRW. In the following, the dual-clutch transmission will be describedonly insofar as is necessary for an understanding of the invention. Thedual-clutch transmission has two input shafts 12, 14, which are arrangedcoaxially to one another and can be connected to the drive source, forexample an internal combustion engine, alternatingly, via twohydraulically actuated multi-plate clutches K1, K2. Input shaft 14 isembodied as a hollow shaft, in which input shaft 12, embodied as a solidshaft, is guided. The two input shafts 12, 14 drive an axially paralleloutput shaft 16 and an intermediate shaft 18 embodied as a hollow shaftvia gear sets of the forward gears and of the reverse gear. The gearsets of forward gears 1 through 7 each have fixed gears and movablegears that can be shifted via hydraulically actuated gear selectors. Thegear selectors may be dual-synchronizer clutches, for example, each ofwhich is capable of switching two neighboring movable gears from aneutral position.

FIG. 2a shows the hydraulic system of the dual-clutch transmission in ahighly simplified block diagram. The hydraulic cylinders 22, 23 of theclutches K1, K2 and of the gear selectors are actuated by means of thehydraulic system. The hydraulic system of FIG. 2a has a high-pressurecircuit H and a low-pressure circuit N. In the high-pressure circuit H,the hydraulic cylinders 22, 23 of the clutches K1, K2 and of the gearselectors, connected therein, can be pressurized via a pressureaccumulator 25 with an accumulator pressure p_(S), which may be in therange of about 30 bar, for example. For this purpose, a main line 27connected to the pressure accumulator 25 leads along clutch paths 30, 31to the clutch hydraulic cylinders 23 and along gear selector paths 32 tothe gear selector hydraulic cylinders 22. Clutch valves or gear selectorvalves 35, 38 are positioned in each of the gear selector paths andclutch paths 30, 31, 32. The clutch valves or gear selector valves 35,38 are controllable in a manner not shown via a central control unit 39.In addition, the control unit 39 is in signal communication withpressure sensors 34. The pressure sensors 34 detect the hydraulicpressure applied to the first clutch K1 and to the second clutch K2.

The hydraulic system further comprises a charge pump 53, which isconnected on the input side to an oil sump 55. The charge pump 53 can beactivated by the control unit 39, via an electric motor 57, to chargethe pressure accumulator 25. In addition, the charge pump 53 is arrangedtogether with a cooling pump 59 on a common drive shaft 60, which isdriven by the electric motor 57. The cooling pump 59 is connected on itsoutput side to a low-pressure line 61, which leads to a distributionvalve 63. When a requirement for cooling exists, the hydraulic fluid canbe conducted to the first and/or to the second clutch K1, K2 andsubsequently back into the oil sump 55, dependent upon the position ofthe distribution valve 63.

In FIG. 2a , the main line 27 of the high-pressure circuit H branchesoff at a branching-off point 65 into a bypass line 67, which isconnected to the low-pressure line 61 of the low-pressure circuit N.Downstream of the branching-off point 65, a check valve 69 ispositioned, which will be described later. Also integrated into thebypass line 67 is an accumulator charging valve 71. The accumulatorcharging valve 71 can be adjusted between the charging position L shownin FIG. 2a and a cooling position K, depending upon the level of theaccumulator pressure p_(S) in the high-pressure circuit H.

The accumulator pressure p_(S) in the high-pressure circuit H acts as acontrol pressure, with which the accumulator charging valve 71 can beadjusted without additional external energy, i.e. automatically. Theaccumulator charging valve 71 is designed to move into the chargingposition L, for example when the accumulator pressure p_(S) in thehigh-pressure circuit H falls below a lower threshold value, for example25 bar. In addition, the accumulator charging valve 71 is automaticallyshifted into its cooling position K when the accumulator pressure p_(S)exceeds an upper threshold value p_(max), for example 28 bar.

During driving operation, actuations of the clutches K1, K2 and of thegear selectors G1 to G4 result in pressure losses. In addition, furtherpressure losses occur due to basic leakage in the high-pressure circuitH, i.e. due to leakage resulting from valve gaps or the like. As aresult, the accumulator pressure p_(S) is reduced during drivingoperation. If the accumulator pressure p_(S) should fall below the lowerthreshold value p_(min) (i.e., if a requirement to charge the pressureaccumulator exists), the accumulator charging valve 71 willautomatically move to its charging position L (FIG. 2). Upon detectionof the requirement to charge the pressure accumulator, the control unit39 will activate the electric motor 57 to a target charging speed. Thisenables the hydraulic charge pump 53 to charge the pressure accumulator25. In such a charging operation, the hydraulic charge pump 53 operatesunder a high pump load and therefore at a correspondingly high actualcurrent consumption I_(max) (FIG. 11). When the accumulator pressurep_(S) exceeds the upper threshold value p_(max) (FIG. 11), i.e. when arequirement to charge the pressure accumulator no longer exists, theaccumulator charging valve 71 automatically moves into its coolingposition K. In the cooling position K, the hydraulic charge pump 53delivers hydraulic oil via the now opened bypass line 67 into thelow-pressure circuit N. At the same time, the high-pressure circuit H isclosed in a pressure-tight manner via the check valve 69. Accordingly,the hydraulic charge pump 53 is no longer operating at a high pump load,but at a reduced pump load and also at a correspondingly reduced actualcurrent consumption I_(min) (FIG. 11).

As mentioned above, upon detection of a requirement to charge thepressure accumulator, the control unit 39 activates the electric motor57 to a target charging speed. For detecting such a requirement tocharge the pressure accumulator, a pressure sensor in the high pressurecircuit H and a position sensor in the accumulator charging valve 71 aredispensed with according to the invention. Instead, the control unit 39is equipped with an analysis unit. The analysis unit is in signalcommunication with a current measuring device 75, which is integratedinto control of the motor and which detects the actual currentconsumption I_(actual) of the electric motor 57, and with a speed sensor77, which detects the actual rotational speed n_(actual) of the electricmotor 57.

FIG. 2b illustrates the basic structure and the functioning of thepressure accumulator 25. According to said figure, the pressureaccumulator 25 is a piston-cylinder unit having an oil chamber 26, whichis connected to the hydraulic lines 27, 31, 32, and a preloaded pressurepiston 27. Preloading is achieved in this example by means of gaspressure applied to the pressure piston 27. Alternatively, preloadingmay be achieved by means of a spring. When the oil chamber 26 iscompletely drained, the pressure piston 27 (indicated in FIG. 2b by adashed line) is pushed with a preload force F_(V) against a stop 29 ofthe pressure accumulator 25. This means that during a filling operation,a hydraulic pressure which is greater than a preload pressure p_(V) thatcorrelates to the preload force F_(V) prevails, to overcome the preloadforce F_(V).

In FIG. 2b , the pressure accumulator 25 is shown in a partially filledstate, in which the hydraulic oil is acting on the pressure piston 27with an accumulator pressure, thereby building up the preload forceF_(V). In the completely drained state, the hydraulic lines 27, 31 arenot pressurized by the pressure accumulator 25. Rather, ambient pressureP_(U) prevails in the hydraulic lines 27, 31, 32. The automatictransmission is ready for operation when all the hydraulic lines 27, 31,32 are filled with hydraulic oil and when the hydraulic pressure in thehydraulic lines 27, 31, 32 is greater than the preload pressure p_(V),specifically by a preset pressure difference, so that the state ofoperational readiness will not be lost again due to basic leakage assoon as the charge pump 53 is switched off.

In FIG. 2a , the control unit 39 includes a diagnostic module 79 withwhich the charging behavior can be checked, more particularly,conditions can be checked to determine whether the actual preloadpressure p_(V) in the pressure accumulator 21 matches a referencepreload pressure p_(VRef) indicated in the specification (i.e.structurally specified). The program blocks required for this areoutlined in FIG. 3. According to said figure, the diagnostic module 79has an analysis unit 80, which compares a temperature-dependent preloadpressure p_(VRef) stored in a characteristic map 83 with an actualaccumulator pressure p_(S)(t_(V)) (FIG. 4), which will be describedlater. The actual accumulator pressure p_(S)(t_(V)) is detected by thepressure sensor 34 at a preload pressure time t_(V), which will bedescribed later. During the diagnostic operation, the clutch valve 35 inone of the clutch paths 30, 31 is constantly open, while the clutchvalve 35 in the other clutch path is closed.

If the pressure accumulator is functioning properly, the actualaccumulator pressure p_(S)(t_(V)) detected at the preload pressure timet_(V) will match the reference preload pressure p_(VRef). In contrast,if a significant preload pressure deviation exists, the analysis unit 80will identify this as a preload pressure fault, which will be stored ina preload pressure fault memory 81 (FIG. 3). If it is determined thatthe pressure accumulator 25 is functioning properly, a further analysisunit 82 (FIG. 4) of the diagnostic module 79 will perform a clutch pathdiagnosis, which will be described later.

In the following, the pressure accumulator diagnosis (i.e., preloadpressure diagnosis) and the clutch path diagnosis will be described inreference to FIGS. 3 and 4: To prepare for pressure accumulatordiagnosis, the oil chamber 26 of the pressure accumulator 25 is drainedcompletely and the actual accumulator pressure p_(S)(t) in the hydraulicsystem is reduced to an ambient pressure p_(U) so that the pressureaccumulator diagnosis can begin at a diagnosis start time t_(S) (FIG.4). The above-described condition for the start of diagnosis is achievedby actuating the hydraulic cylinders 22, 23 of the clutches K1, K2 andthe gear selectors G1 to G4, as indicated in the graph illustratingtravel distance at the top of FIG. 4. Accordingly, the hydrauliccylinders 22, 23 are activated intermittently by supplying power to therespective clutch valves or gear selector valves 35, 38 until, due tothe removal of hydraulic fluid associated with the hydraulic cylinderactuation, the accumulator pressure p_(S) detected by the pressuresensor 34 is reduced to the ambient pressure p_(U). The existence ofsuch an ambient pressure p_(U) can be detected by the pressure sensor34. Alternatively, position sensors 93 in the hydraulic cylinders 22, 23may be used to determine whether or not the respective hydrauliccylinder 22, 23 is still traveling a travel distance s (FIG. 4). If not,it will be concluded that an ambient pressure p_(U) exists in thehydraulic system.

Diagnostic charging operation, in which the hydraulic charge pump 53 isoperated at a constant charging speed n_(L) (FIG. 4, lower graph), thenbegins at time t_(S) (FIG. 4). First, for example, the pressure sensor34 located in the first clutch path 31 detects the actual accumulatorpressure profile p_(S)(t), as represented in the middle graph in FIG. 4.As illustrated by said graph, the accumulator pressure p_(S) increasesuntil the preload pressure time t_(V) at which the actual accumulatorpressure p_(S)(t_(v)) detected by the pressure sensor 34 has reached thepressure accumulator preload pressure p_(V).

As was stated above, if the pressure accumulator is functioningproperly, the actual accumulator pressure p_(S)(t_(V)) detected at thepreload pressure time t_(V) (accounting for temperature dependencies)will be identical to a reference preload pressure p_(VRef). If theactual accumulator pressure p_(S)(t_(V)) detected at the preloadpressure time t_(V) is found to deviate significantly from the referencepreload pressure p_(VRef), the analysis unit 80 will diagnose a preloadpressure fault. As diagnostic charging operation continues, after thepreload pressure time t_(V), the oil chamber 26 of the pressureaccumulator 25 is filled, specifically by displacement of the pressurepiston 27.

As is clear from the middle graph of FIG. 4, during diagnostic chargingoperation the actual accumulator pressure profile p_(S)(t) rises with asteep pressure gradient pi until the preload pressure p_(V) is reachedin the pressure accumulator 25 (i.e. up to the preload pressure timet_(V)). Afterward (i.e. after the preload pressure time t_(V)), incontrast, the actual accumulator pressure profile p_(S)(t) rises withonly a shallower pressure gradient {dot over (p)}₂. This characteristiccharging curve for the pressure accumulator 25 is used as follows todetermine the preload pressure time t₂: The analysis unit 80 detects thepressure gradients {dot over (p)}₁, {dot over (p)}₂ of the actualaccumulator pressure profile p_(S)(t). When a significant gradientchange between the pressure gradients {dot over (p)}₁ and {dot over(p)}₂ is detected, the analysis unit 80 identifies this as the preloadpressure time t_(V).

If no preload pressure fault is detected in the above preload pressurediagnosis, this will be followed immediately by the clutch pathdiagnosis: For this purpose, the diagnostic charging operation carriedout during the pressure accumulator diagnosis is simply continued untilthe pressure sensor 34 reaches an upper threshold value p_(max) (FIG. 4,middle graph). In the middle graph of FIG. 4, the upper threshold valuep_(max) lies above the preload pressure p_(V) of the pressureaccumulator 25 by a pressure difference Δp. When the diagnostic chargingoperation is completed, a second analysis unit 82 compares a pressuregradient {dot over (p)}₃ of the actual accumulator pressure profilep_(S)(t) with a reference pressure gradient {dot over (p)}_(Ref), whichis stored on a temperature-dependent basis in a characteristic map 84(FIG. 3) in the diagnostic module 79. Based upon this comparison, theanalysis unit 82 determines whether a fault-free or a faultyleakage-induced pressure decrease is present in the actual accumulatorpressure profile p_(S)(t).

It should be emphasized that the clutch path diagnosis is performed onlyif the analysis unit 80 does not detect a preload pressure fault. If thepressure accumulator 25 is fault-free, any faulty leakages can beunambiguously assigned to the clutch path 31. Both during the pressureaccumulator diagnosis and during the clutch path diagnosis, the pressurecontrol valve 36 located in the connecting line 37, which connects themain line 27 to the gear selector paths 32, is closed.

To validate the results obtained in the preload pressure/clutch pathdiagnosis, the diagnostic operation described above in reference to thefirst clutch path 31 can be performed twice, specifically as part of afirst partial diagnosis A using the pressure sensor 34 located in thefirst clutch path 31 and with the clutch valve 35 in the second clutchpath 32 closed. The above diagnostic operation can then be performed aspart of a second partial diagnosis B, specifically with the pressuresensor 34 located in the second clutch path 30 and with the clutch valve35 in the first clutch path 31 closed.

If the same fault is detected in both the first partial diagnosis A andthe second partial diagnosis B, the diagnostic module 79 can diagnose apressure accumulator fault and can rule out a clutch path fault withhigh probability. If different fault results are obtained, thediagnostic module 79 can diagnose a leakage fault in one of the twoclutch paths 30, 31.

FIG. 5 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for a gear selector pathdiagnosis. The gear selector path diagnosis is performed as a follow-ondiagnosis immediately following the clutch path diagnosis (FIG. 3) underthe condition that at least one clutch path 30, 31 is diagnosed ashaving fault-free leakage during the clutch path diagnosis. The pressuresensor 34 of the clutch path 30, 31 that is classified as fault-free(hereinafter referred to as the reference clutch path) is used for thegear selector path diagnosis illustrated in FIGS. 5 and 6.

As is clear from FIG. 5, the diagnostic module 79 has a third analysisunit 85, at the signal input of which an actual accumulator pressurep_(S)(t) detected by the pressure sensor 34 and an actual accumulatorpressure gradient {dot over (p)} are applied. The analysis unit 85checks the leakage behavior of each of the gear selector paths 32separately. Any leakage faults that are detected are stored in the faultmemory 87.

In the following, the gear selector path diagnosis will be described inreference to FIGS. 5 and 6: The diagnostic module 79 begins by openingthe clutch valve 35 located in the reference clutch path 30, in order todetect the actual accumulator pressure profile p_(S)(t). The pressurecontrol valve 36 in the connecting line 37 of the hydraulic system isalso opened, to establish a pressure connection between the pressuresensor 34 located in the reference clutch path 30 and the gear selectorpaths 32. A diagnostic charging operation is then performed byactivating the hydraulic charge pump 53. During the diagnostic chargingoperation, the actual accumulator pressure p_(S)(t) is increased up tothe upper threshold value p_(max) (FIG. 6) at the end time t_(off). Whenthe diagnostic charging operation has ended, i.e. at the end timet_(off) (FIG. 6), the pressure sensor 34 detects a pressure gradient{dot over (p)}_(K+G) of the accumulator pressure profile p_(S)(t) duringa measurement time interval Δt_(M). The analysis unit 85 compares thepressure gradient {dot over (p)}_(K+G) with a reference pressuregradient p_(Ref) and analyzes whether a fault-free or a faulty pressuredecrease (i.e. a gear selector leak) is present in the accumulatorpressure profile p_(S)(t).

As shown in FIG. 2a , each of the gear selector valves 35 located in thegear selector paths 32 can be adjusted between a closed valve position Sand two flow-through valve positions D1, D2. The gear selector pathdiagnosis is performed for each of the flow-through valve positions D1and D2 separately in the gear selector path 32 to be tested. This meansthat in each gear selector path 32, the gear selector diagnosis iscarried out both with the gear selector valve 38 in the firstflow-through valve position D1 and with the gear selector valve 38 inthe second flow-through valve position D2. In contrast, the gearselector valves 38 in the remaining gear selector paths 32 remainswitched to the closed valve position S, in order to increase measuringaccuracy in the diagnosis of the gear selector path 32 being tested. Thepressure gradient {dot over (p)}_(K+G) detected in the measurement timeinterval Δt_(M) by the pressure sensor 34 therefore reflects thecollective pressure decrease both in the reference clutch path 30 and inthe gear selector path 32 being tested, the gear selector valve 38 ofwhich is switched to one of the two flow-through positions D1, D2.

The reference pressure gradient p_(Ref) is read out from acharacteristic map database, e.g. from the characteristic map database83 already shown in FIG. 3. In this case, the readable referencepressure gradient p_(Ref) would correspond to a fault-free basic leakageof the reference clutch path 30. In the analysis unit 85, in addition tothe detection of the pressure gradients p_(K+G), absolute pressurevalues are also detected, i.e. the actual accumulator pressurep_(S)(t_(Start)) at the start time t_(Start) and the actual accumulatorpressure p_(S)(t_(End)) at the measurement end time t_(End) of themeasurement time interval Δt_(M). In this case, if the conditions aremet that, first, there is a sufficiently large accumulator pressuredifference between the start time t_(Start) and the end time t_(End),and second, the pressure gradient {dot over (p)}_(K+G) is equal to thereference pressure gradient {dot over (p)}_(Ref), the analysis unit 85will diagnose a fault-free gear selector path 32.

FIG. 7 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for an accumulator volumediagnosis. The accumulator volume diagnosis is performed as a follow-ondiagnosis immediately following the gear selector diagnosis (FIGS. 5 and6), under the condition that in the gear selector diagnosis, at leastone gear selector path 32 of gear selectors G1 to G4 has been diagnosedas fault-free and can thus be used as a reference gear selector path inthe accumulator volume diagnosis.

As is clear from FIG. 7, the diagnostic module 79 has an analysis unit89 which, in a comparator block 97, compares a hydraulic fluid removalV_(E) determined during the accumulator volume diagnosis with areference accumulator volume V_(ref). If a significant deviation isfound, an accumulator volume fault is diagnosed, which is stored in thefault memory 91. The reference accumulator volume V_(ref) can be readout from an accumulator volume characteristic map in a database, inwhich the reference values are stored on a temperature-dependent basis.

As is further clear from FIG. 7, the analysis unit 89 is in signalcommunication with a position sensor 93 of the gear selector hydrauliccylinder 22 located in the reference gear selector path 32. During theaccumulator volume diagnosis, the gear selector valve 38 in thereference gear selector path 32 is actuated, and the position sensor 93detects the travel distances Δs of the gear selector hydraulic cylinder22. These are integrated in a travel distance integrator 95 to obtain atotal travel distance s_(total). In a converter block 96, the totaltravel distance s_(total) is converted to a total displacement volumeV_(S). To the total displacement volume V_(S), a hydraulic fluid leakagevolume V_(L) that flows out during the accumulator volume diagnosis isadded. The resulting hydraulic fluid removal V_(E) is forwarded to theaforementioned comparator block 97.

The accumulator volume diagnosis is performed as follows: First, thepressure accumulator 25 is filled completely with hydraulic fluid in adiagnostic charging operation, i.e. it is charged to an upper thresholdvalue p_(off), which in FIG. 8 is reached at the switch-off timet_(off). The reference hydraulic cylinder 22 is then actuatedintermittently within a diagnosis time interval Δt_(D), beginning at astart time t_(start) (which in FIG. 8 coincides with the switch-off timet_(off)) and continuing until, as a result of the leakage volume V_(L)and the displacement volume V_(S) removed from the hydraulic system, anambient pressure p_(U) is present in the hydraulic system. Rather thanbeing measured via a pressure sensor, the ambient pressure p_(U) isdetected indirectly in the diagnostic module 79, specifically at the endtime t_(end) (FIG. 8) of the diagnosis time interval Δt_(D), when,despite the flow-through valve position D1, D2 of the reference controlvalve 35, the position sensor 93 no longer detects any further traveldistance Δs.

During the pressure accumulator volume diagnosis, one of the clutchpaths 30, 31 as the reference clutch path, along with the reference gearselector path 32 that leads to the reference hydraulic cylinder 22, ispressurized with the accumulator pressure p_(S) prevailing in thehydraulic system. In contrast, the hydraulic cylinders 22 of the othergear selector paths 32 and of the other clutch path are decoupled fromthe accumulator pressure p_(S).

The leakage volume V_(L) can be determined based upon the pressuregradients in the clutch path 30 and at the reference gear selector 22,detected during the preceding diagnoses (e.g. the pressure gradient {dotover (p)}_(K+G) from the gear selector path diagnosis according to FIGS.5 and 6). The pressure gradient {dot over (p)}_(L) is multiplied in theanalysis unit 89 by the diagnosis time interval Δt_(D). The resultingpressure difference Δp_(L) is converted in a converter 98 to the leakagevolume V_(L).

FIG. 9 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for a switchover pointdiagnosis of the accumulator charging valve 71. The switchover timingdiagnosis is performed as a follow-on diagnosis immediately followingthe accumulator volume diagnosis (FIGS. 7 and 8), under the conditionthat a plausible accumulator volume of the pressure accumulator 25 wasdiagnosed in the accumulator volume diagnosis.

As is clear from FIG. 9, the diagnostic module 79 has an analysis unit105 with which, as part of the switchover timing diagnosis, a check ismade to determine whether a first switchover time t_(U1), at which theaccumulator charging valve 71 switches automatically from its chargingposition L to its non-charging position K, and a second switchover timet_(U2), at which the accumulator charging valve 71 switchesautomatically from its non-charging position K to its charging position,are plausible. For this purpose, the analysis unit 105 determineswhether, at the first switchover time t_(U1), the actual accumulatorpressure p_(S)(t) is within range of the upper pressure threshold valuep_(max). In addition, the analysis unit 105 determines whether at thesecond switchover time t_(U2), the actual accumulator pressure p_(S)(t)is within range of the lower pressure threshold value p_(min).

For detecting the two switchover times t_(U1) and t_(U2), the currentmeasuring device 75 of the electric motor 57 is used. The currentmeasuring device 75 detects the actual current consumption I_((t)) ofthe electric motor 57. In this process, the time of a change from a highcurrent consumption I_(max) to a low current consumption I_(min) isdefined by the control unit 39 as the first switchover time t_(U1). Thetime of a change from the low current consumption I_(min) to the highcurrent consumption I_(max) is defined as the second switchover timet_(U2).

The clutch path pressure sensor 34 is used to detect the actualaccumulator pressure p_(S)(t). In FIG. 11, the measuring range Δp_(meas)of said sensor (FIG. 11) lies outside of, i.e. below the pressurethreshold values p_(max) and p_(min). Thus, a detection of the actualaccumulator pressure p_(S) immediately at the two switchover timest_(U1) and t_(U2) is not possible because the actual accumulatorpressure at the two switchover times lies outside of the measuring rangeΔp_(meas).

In FIG. 9, the actual accumulator pressure p_(S)(t) is determined at theswitchover times t_(U1) and t_(U2) by estimation, specifically with theaid of an extrapolation block 107. In the extrapolation block 107, basedupon measured pressure values p_(a)(t_(a)) and p_(b)(t_(b)) in theaccumulator pressure profile that are within the pressure sensormeasuring range (Δp_(meas)), a time frame Δt_(target) is estimated. Ifthe accumulator charging valve is operating properly, the firstswitchover time t_(U1) will lie within the time frame Δt_(target). InFIGS. 9 and 11, the time frame Δt_(target) is bounded by the two timest₁ and t₂. In the subsequent comparator block 108 it is determinedwhether the first switchover time t_(U1) lies within or outside of thetime frame Δt_(target). If the first switchover time t_(U1) lies outsideof the time frame Δt_(target), a fault will be diagnosed, which will bestored in the fault memory 109.

FIG. 9 shows only a partial diagnosis in the program blocks, in which acheck is made to determine whether or not the first switchover timet_(U1) lies within the time frame Δt_(target). In the same manner, theanalysis unit 105 checks to determine whether or not the secondswitchover time t_(U2) lies within an estimated time frame.

FIG. 10 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for a valve spreaddiagnosis. The valve spread diagnosis is performed as a follow-ondiagnosis immediately following the switchover timing diagnosis (FIG. 9)under the condition that at least one plausible switchover time t_(U1)of the charging accumulator valve 71 has been identified in theswitchover timing diagnosis.

In FIG. 10, the diagnostic module 79 has an analysis unit 99 which, in avalve spread diagnosis, determines the actual valve spread Δp_(actual)between the lower pressure threshold value p_(min) and the upperpressure threshold value p_(max). A comparator block 101 of the analysisunit 99 compares the actual valve spread Δp_(actual) with a target valvespread Δp_(Ref). If a significant deviation is found, a fault isdiagnosed and is stored in the fault memory 103.

To determine the actual valve spread Δp_(actual), the analysis unit 99defines a diagnosis time interval Δt_(D). The diagnosis time intervalΔt_(D) begins at the first switchover time t_(U1) and ends at thesubsequent second switchover time t_(U2). Within the above-defineddiagnosis time interval Δt_(D), the diagnostic module 79 activates areference hydraulic cylinder 22, which according to FIG. 11 is switchedback and forth constantly, i.e. intermittently, during the diagnosistime interval Δt_(D). Due to the actuation of the reference hydrauliccylinder 22 and due to a system-inherent hydraulic system leakage, anaccumulator pressure decrease Δp_(E) that corresponds to the actualvalve spread Δp_(actual) occurs during the diagnosis time intervalΔt_(D).

The accumulator pressure decrease Δp_(E), i.e. the actual valve spreadΔp_(actual), is determined using the program blocks shown in FIG. 10, asfollows: From the position sensor 93, the piston travel distances Δs areintegrated in an integrator 94 during the diagnosis time interval Δt_(D)to obtain a total travel distance s_(total). This is then used in aconverter block 95 to calculate the pressure decrease Δp_(B) associatedwith the gear selector actuation. The pressure decrease Δp_(B)associated with the gear selector actuation is added in a summationelement to the leakage-induced pressure decrease Δp_(L), which gives theaccumulator pressure decrease Δp_(E) during the diagnosis time intervalΔt_(D). The leakage-induced pressure decrease Δp_(L) in the referencehydraulic cylinder 22 has already been determined in previous diagnoses.

As shown in FIG. 2a , connected upstream of the two clutch paths 30, 31is a safety valve 28 that can be activated by the electronic controlunit 39. The safety valve 28 can be actuated between a closed positionand a flow-through position. In the closed position, the two clutchpaths 30, 31 are pressure-decoupled from the pressure accumulator 25. Inthe flow-through position, the two clutch paths 30, 31 can bepressurized with the accumulator pressure p_(S). If the control unit 39detects a malfunctioning of the clutch valve 35 in at least one of theclutch paths 30, 31, the safety valve 28 can be adjusted to its closedposition for safety reasons. During normal driving operation, the safetyvalve 28 is constantly in its flow-through position.

FIG. 12 is a simplified block diagram showing the program blocks of thediagnostic module 79 that are required for a safety valve diagnosis. Thesafety valve diagnosis can be performed independently of otherdiagnostic steps. During the safety valve diagnosis, the safety valve 28is switched from the flow-through position to the closed position at adiagnosis start time t_(Start) (FIG. 13), thereby creating an actualpressure decrease Δp_(actual) downstream of the safety valve 28. Thediagnostic module 79 has an analysis unit 111 which compares this actualpressure decrease Δp_(actual) with a target pressure decreaseΔp_(target). If a significant deviation is found, a fault is diagnosedand stored in a safety fault memory 113.

The aforementioned clutch pressure sensor 34 can be used to detect theactual pressure decrease Δp_(actual).

In the following, the performance of the safety valve diagnosis will bedescribed in reference to FIGS. 12 and 13: For proper measuringaccuracy, the hydraulic pump 53 is activated to a constant speedn_(test) to ensure that there is sufficient accumulator pressure p_(S)in the high-pressure circuit H, which according to FIG. 13 moves betweenthe upper pressure threshold value p_(max) and the lower pressurethreshold value p_(min). The clutch valve 35 of a reference clutch path30 or 31 is adjusted to its flow-through position prior to theaforementioned start time t_(Start) by a time difference Δt, so that thepressure sensor 34 between the clutch valve 35 and the clutch hydrauliccylinder 23 can detect the actual pressure decrease Δp_(actual). Duringthe time difference Δt, rather than reading out the hydraulic pressureactually present at the clutch hydraulic cylinder 22 to the analysisunit 111 (FIG. 12), the pressure sensor 34 reads out an upper limitingpressure of the measuring range Δp_(meas).

At the diagnosis start time t_(Start), the safety valve 28 is switchedfrom its flow-through position D to its closed position S. The resultingpressure decrease p_(actual) is detected by the pressure sensor 34 andis compared in the analysis unit 111 with the target pressure decrease.

FIG. 14 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for the delivery volumeflow diagnosis. The delivery volume flow diagnosis is performed as afollow-on diagnosis immediately following the accumulator volumediagnosis (FIGS. 7 and 8), under the condition that a plausibleaccumulator volume of the pressure accumulator 25 is detected in theaccumulator volume diagnosis.

As is clear from FIG. 14, a gear selector hydraulic cylinder 22, whichis connected to the pressure sensor 25 via the gear selector 32, is usedfor the diagnosis. Upstream of the gear selector hydraulic cylinder 22is a gear selector valve 38, which can be actuated by the control unit39 to adjust the hydraulic pressure applied to the gear selectorhydraulic cylinder 22. The gear selector valve 38 can be adjustedbetween two flow-through positions D1, D2 in order to displace a piston33 in opposing piston strokes over the indicated travel distances s₁, s₂and at piston speeds {dot over (s)}₁, {dot over (s)}₂ in the hydraulicactuating cylinder 22. In FIG. 14, the piston 33 divides the hydrauliccylinder into a piston rod-side working chamber and a working chamberopposite the first, the two of which are connected via hydraulic controllines 41 to the gear selector valve 38. A gear selector G1, not shown,can be actuated by means of the piston rod 43 of the gear selectorhydraulic cylinder 22. With such a gear selector actuation, theelectronic control unit 39 controls the gear selector valve 38 in aknown manner into one of the flow-through positions D1, D2, in order tomove the piston rod. The piston stroke is associated with a hydraulicfluid removal V₁, V₂ (displacement volume) from the hydraulic system.Because the internal geometry of the gear selector hydraulic cylinder 22is known, the respective displacement volume V₁, V₂ is known. Alsoprovided is a position sensor 93, with which the piston speed {dot over(s)}₁, {dot over (s)}₂ in the respective piston stroke can be detected.

In the following, the delivery volume flow diagnosis will be describedin reference to FIGS. 14 and 15: First, the hydraulic pump 53 is firstdeactivated within a pressure reduction time interval Δt_(R) (FIG. 15),while at the same time, the gear selector valve 38 is actuatedintermittently by the electronic control unit 39, as shown in the middletravel distance graph of FIG. 15. During the pressure reduction timeinterval Δt_(R), the gear selector valve 38 is actuated so as to movethe gear selector hydraulic cylinder 22 back and forth until, as aresult of leakage-induced hydraulic fluid removal and as a result ofactuation-induced hydraulic fluid removal (i.e., displacements V₁, V₂),the accumulator pressure p_(S)(t) is reduced to the ambient pressurep_(U). In this state, the accumulator 25 has been completely drained.This is followed immediately by the start (t_(Start)) of a diagnosistime interval Δt_(D). During the diagnosis time interval Δt_(D) acharging operation of the hydraulic pump 53 is carried out, in which thepump is actuated at different test speeds n₁ and n₂. At the same time,the control valve 35 is adjusted intermittently between its flow-throughpositions D1, D2. This causes the piston 33 in the gear selectorhydraulic cylinder 22 to move back and forth in the gear actuatorhydraulic cylinder 22 in opposing piston strokes over piston traveldistances s₁, s₂ and at piston speeds {dot over (s)}₁, {dot over (s)}₂.

The position sensor 93 detects both the individual travel distances s₁,s₂ per piston stroke and the piston speeds {dot over (s)}₁, {dot over(s)}₂ per piston stroke. In addition, the number a (FIG. 14) of pistonstrokes during the diagnosis time interval Δt_(D) is detected. Thesedata are forwarded to the signal input of a converter block 115, inwhich an average piston speed {dot over (s)}_(average) is calculatedfrom the number a of detected piston strokes. From the average pistonspeed {dot over (s)}_(average), an actual delivery volume flowV_(actual) is calculated in the converter block 115. In an analysis unit113 which is connected in terms of signal communication downstream, theactual delivery volume flow V_(actual) is compared with a targetdelivery volume flow V_(target), factoring in the respective test speedn₁ and n₂ during the diagnosis time interval Δt_(D). If a significantdeviation is detected in the analysis unit 113, a fault is detected,which is stored in the fault memory 117.

As is clear from FIG. 16, all of the fault memories 81, 83, 87, 91, 103,109, 117 are in signal communication with an analysis unit 120, intowhich the fault signals generated in the fault memories can be read. Inthe analysis unit 120, an analysis matrix is stored, in which the faultsignals from the fault memories 81, 83, 87, 91, 103, 109, 117 aremerged.

For a comprehensive hydraulic system diagnosis, the analysis unit 120uses the analysis matrix to analyze all the fault signals incombination. Thus in the analysis unit 120, a comparison of faultsignals with acceptable, i.e. fault-free, functional diagnoses isfinally performed, thereby enabling a qualified appraisal of thecomponents installed in the hydraulic system. This appraisal is possiblewithout dismantling of the hydraulic system and without external testingequipment/measuring technology. By testing components in the installedstate (in the vehicle), a shortening of repair and maintenance times, areliable detection of defective components, a decrease in repairs thatmust be repeated, and a savings on analysis test bench capacities arepossible without the effort associated with dismantling.

The invention claimed is:
 1. A hydraulic system for an automatictransmission of a motor vehicle, comprising: a high-pressure circuit, inwhich a pressure accumulator, at least one clutch, and gear selectorsare connected, and comprising: a low-pressure circuit for cooling the atleast one clutch, wherein the high-pressure circuit and the low-pressurecircuit have at least one hydraulic pump, which can be driven by anelectric motor, said hydraulic system also having a control unit, whichactivates the electric motor of the hydraulic pump when a requirement tocharge the pressure accumulator is identified, wherein the high-pressureand low-pressure circuits are connected via a bypass line to anintegrated accumulator charging valve, which in a non-charging positionfluidically connects the hydraulic pump to the low-pressure circuit andin a charging position fluidically connects the hydraulic pump to thehigh-pressure circuit, wherein the accumulator charging valve switchesautomatically from the charging position to the non-charging position ata first switchover time if the accumulator pressure in the high-pressurecircuit exceeds an upper pressure threshold value (p.sub.max), andswitches automatically from the non-charging position to the chargingposition at a second switchover time if the accumulator pressure dropsbelow a lower pressure threshold value, wherein the control unit has adiagnostic module, by which a valve spread diagnosis is performed, inwhich an actual valve spread between the lower and upper pressurethreshold values can be determined, and the diagnostic module has ananalysis unit, which compares the actual valve spread with a targetvalve spread and, if a significant deviation is found, diagnoses afault, which can be stored in a valve spread fault memory.
 2. Thehydraulic system according to claim 1, wherein to determine the actualvalve spread, the analysis unit specifies a diagnosis time interval,which begins at the first switchover time and ends at the secondswitchover time, and during the diagnosis time interval, an accumulatorpressure decrease that corresponds to the actual valve spread occurs,both as a result of actuation of a reference hydraulic cylinder and as aresult of hydraulic system leakage.
 3. The hydraulic system according toclaim 2, wherein for determining the accumulator pressure decreaseduring the diagnosis time interval, the reference hydraulic cylinder hasa position sensor, which detects the piston travel distances occurringwith gear selector actuations, and during the diagnosis time intervalthe diagnostic module integrates the piston travel distances to obtain atotal travel distance, from which the diagnostic module calculates thepressure decrease associated with the gear selector actuations, and fromthe sum of the pressure decrease associated with the gear selectoractuations and the leakage-induced pressure decrease, the analysis unitdetermines the accumulator pressure decrease.
 4. The hydraulic systemaccording to claim 1, wherein when the accumulator charging valve is inthe charging position, the hydraulic pump operates at a high pump loadand a high current consumption, and when the accumulator charging valveis in the non-charging position, the hydraulic pump operates at a lowpump load and a low current consumption.
 5. The hydraulic systemaccording to claim 4, wherein the control unit is assigned a currentmeasuring device which can detect the actual current consumption of theelectric motor, and the control unit identifies the time of the changefrom the high current consumption to the low current consumption as thefirst switchover time and identifies the time of the change from the lowcurrent consumption to the high current consumption as the secondswitchover time.
 6. The hydraulic system according to claim 2, whereinthe reference hydraulic cylinder is a gear selector hydraulic cylinder,and in a reference gear selector path leading from the pressureaccumulator to the reference hydraulic cylinder, a gear selector valveis arranged, which can be controlled by the control unit and with whichthe rate of flow through the reference hydraulic cylinder can beadjusted.
 7. The hydraulic system according to claim 1, wherein thevalve spread diagnosis is performed as a follow-on diagnosis following aswitchover timing diagnosis, in which it is determined whether at thefirst switchover time, the actual accumulator pressure is within rangeof the upper pressure threshold value, wherein a fault can be stored ina switchover time fault memory.
 8. The hydraulic system according toclaim 7, wherein in at least one clutch path leading from the pressureaccumulator to a clutch hydraulic cylinder, a clutch valve is arranged,which can be controlled by the control unit and with which the hydraulicpressure applied to the clutch hydraulic cylinder can be adjusted, andthe electronic control unit is assigned a pressure sensor, with whichthe hydraulic pressure applied to the clutch hydraulic cylinder can bedetected.
 9. The hydraulic system according to claim 8, wherein theclutch path pressure sensor is used to detect the actual accumulatorpressure during the switchover timing diagnosis.
 10. The hydraulicsystem according to claim 9, wherein a measuring range of the pressuresensor lies outside of the upper pressure threshold value, and on thebasis of measured pressure values that lie within the pressure sensormeasuring range, an analysis unit estimates a time frame within whichthe first switchover time will lie if the valve is functioning properly,that if the first switchover time lies outside of this time frame, afault is diagnosed, which can be stored in a fault memory.
 11. Thehydraulic system according to claim 2, wherein in the components thatare pressurized with the accumulator pressure during the diagnosis timeinterval, a leakage-induced pressure decrease occurs.
 12. The hydraulicsystem according to claim 11, wherein the leakage-induced pressuredecrease occurs based upon previous diagnoses or leakage measurements.13. The hydraulic system according to claim 1, wherein the switchovertiming diagnosis is performed as a follow-on diagnosis following anaccumulator volume diagnosis and a gear selector path diagnosis.
 14. Thehydraulic system according to claim 13, wherein the diagnostic modulecan perform an accumulator volume diagnosis, in which an analysis unitof the diagnostic module compares the actual accumulator volume of thepressure accumulator with a reference accumulator volume of the pressureaccumulator and if a significant deviation is found, diagnoses a fault,which can be stored in an accumulator volume fault memory.